The present invention relates to antenna systems and more particularly to a waveguide antenna assembly for transmitting and/or receiving circularly-polarized signals.
Within the field of waveguide antenna systems, dual waveguide polarizers are known to provide the capability of transmitting and /or receiving left hand circularly-polarized (LHCP) signals and right hand circularly-polarized (RHCP) signals over the same frequency band. The ability to communicate both signal types over the same frequency band effectively doubles the system's communication capability compared to a linear antenna system.
FIG. 1 illustrates one such waveguide polarizer 100 for transmitting and receiving orthogonal LHCP and RHCP signals as described in A Wide-Band Square-Waveguide Array Polarizer, IEEE Transactions on Antennas and Propagation, Vol. AP21, No. 3, May 1973. The waveguide polarizer 100 includes a single aperture waveguide 120, a septum-loaded waveguide 140, and a dual aperture waveguide 160 coupled inline. The single aperture waveguide 120 includes walls 102 which defines a waveguide cavity 122 for transmitting an outgoing or receiving an incoming signal. The septum-loaded waveguide 140 includes a septum 148, which may be stepped or tapered and which forms waveguide channels 144 and 146. The septum dimensions are typically based upon the center frequency of operation (or wavelength) and scaled to the dimensions needed. Typically, the septum 148 is designed as having a infinitesimally small thickness (usually about 1-2% of the wavelength at center frequency) and can deteriorate the polarizer's performance if it is fabricated too thickly. In the conventional polarizer of FIG. 1, the septum is 0.014.lambda. thick to introduce only minimal error into the measured response.
The dual aperture waveguide 160 includes LHCP and RHCP signal ports 164 and 166 for sensing or launching the LHCP or RHCP signals, respectively, during reception or transmission. Probes may be located within these ports to facilitate sensing or exciting the LHCP and RHCP signals. A common wall 168 extends from septum 148 to separate the LHCP and RHCP signal ports 164 and 166. A feedhorn (not shown) is connected to the single aperture waveguide 120 for launching or receiving the LHCP or RHCP signals.
As known in the art, the dimensions of both the single aperture waveguide 122 and the interfacing circular feedhorn (not shown) are critical to provide a good impedance match at the polarizer/feedhorn interface and to ensure proper signal isolation between the orthogonal LHCP and RHCP signal ports. In conventional systems, such as those shown in U.S. Pat. No. 3,955,202 to Young, similar geometry feedhorns and polarizers are used, i.e., circular feedhorns are typically employed with circular polarizers and rectangular feedhorns with rectangular polarizers.
The above described polarizer/feedhorn assemblies suffer from several important disadvantages. Firstly, the conventional waveguide polarizer suffers from the disadvantage of extremely small and difficult to manufacture septum dimensions as the center frequency of operation increases beyond X-band (10 GHz). For instance, the conventional waveguide polarizer of FIG. 1 illustrates a septum 148 having a thickness of 0.014.lambda. and a first step height of 0.080.lambda.. Using this design, a waveguide polarizer operating in the Ka-band (18-20 GHz) would require a septum thickness of 0.039 mm and a first step height of 0.221 mm. Waveguide polarizers of these minute dimensions are exceedingly difficult and costly to manufacture and are extremely unreliable due to the fragility of their small components. As communication systems increase in operational frequency, these encumbrances become more even more pronounced.
Secondly, the prior art assemblies suffer from the limitation that the polarizer and feedhorn are of similar geometries, i.e. rectangular feedhorns matched to rectangular polarizers and circular feedhorn matched to circular polarizers. Rectangular waveguide polarizers are preferred over circular waveguide polarizers since rectangular polarizers are more easily matched to widely used rectangular waveguide systems. However, circular feedhorns are preferred since they exhibit less signal loss compared to rectangular feedhorns. Implementing a circular feedhorn with a rectangular waveguide polarizer could provide several advantages but a method teaching their combination has not been taught in the prior art.
What is needed is a new waveguide polarizer design which can operate at high frequencies but which can also be easily manufactured. Further needed is a waveguide assembly design and matching technique for interfacing a circular feedhorn with a rectangular polarizer assembly to provide high signal isolation between orthogonal signals.